In a wind power station, a wind rotor driven by wind rotates the rotor of a generator to produce electrical energy which is supplied to a power grid. A rotational speed of the wind rotor generally changes with different wind conditions. Further, the speed of the wind rotor is typically much lower than a suitable operation speed of the generator. One known way to cope with these conditions is to arrange a transmission between the wind rotor and the generator, and to use an asynchronous electrical machine as the generator. The transmission increases the speed of the wind rotor at a fixed ratio to have a speed of the generator rotor in a suitable operation range of the generator. The asynchronous generator allows for supplying alternating current to the power grid at a fixed grid frequency over a range of rotational speeds of the generator rotor. This range may even be expanded either by providing a multi pole electrical machine as the asynchronous generator and switching between different pole groups, or by providing an asynchronous double-fed electrical machine as the generator and controlling electrical quantities fed to the rotor of the generator by power converters electrically arranged between the generator rotor and the power grid. In another type of wind power stations the suitable operation range of a generator is expanded by electrically arranging frequency converters between the stator of the generator and the power grid. In this case, the generator may even be a synchronous electrical machine, but it is a clear disadvantage that the full electrical energy supplied by the generator has to be frequency converted which requires high technical efforts. Under most conditions, a direct coupling of the stator of the generator to the power grid is preferred in which an alternating current supplied by the generator has the same grid frequency as an alternating voltage provided by the power grid. With any asynchronous electrical machine as the generator, the power grid to which the generator is coupled provides the voltage for operating the generator. If this voltage breaks down because of a failure of the power grid the generator can no longer be operated, even if there is sufficient wind to be converted into electrical energy. Furthermore, a sudden breakdown of the voltage supplied by the power grid may result into a torque reversal at the rotor of the asynchronous generator. I.e. at that moment at which the voltage supplied by the power grid brakes down, the rotor which was up to that time driven by an output shaft of the transmission transferring the torque applied by the wind to the wind rotor suddenly applies a driving torque to the output shaft of the transmission. This driving torque may be present only for a short time but it results into a shock impact on the transmission considerably affecting the lifetime of the transmission. As a result the total lifetimes of transmissions of wind power stations connected to power grids showing a comparatively high number of accidental breakdowns is strongly reduced, even if the transmissions are perfectly matched to maximum loads exerted under strong wind conditions.
In the art of uninterrupted power supply (UPS) systems, it is known to provide a choke, i.e. a high inductance between a load to be uninterruptedly supplied with electrical power and a power grid, breakdowns of which are encountered. A generator coupled to a fly wheel and/or a combustion engine as an auxiliary power source is connected to the choke closer to the load side than to the grid side. In this arrangement the choke decouples the load from the power grid in such a way that a breakdown of the voltage provided by the power grid is retarded for such a long time that a switch on the grid side of the choke can be opened to disconnect the choke and the load from the grid so that the generator which was previously in no-load operation may now supply electrical energy to the load without loosing the electrical energy to the broken down power grid. If the choke is dimensioned and connected appropriately, it is not noticed at the load that the power supply was switched from the power grid to the generator and that the load was indeed cut off from the power grid. As long as the power grid is working, the generator is driven by the voltage supplied by the power grid as a motor but nearly free of external forces as soon as the generator rotates with a rotational speed corresponding to the grid frequency. However, the generator can also be used to adjust a phase angle phi between an alternating voltage provided by the power grid and an alternating current taken from the power grid. Generally, deviations of cos phi from 1 defining the reactive power taken from the power grid which depend on the properties of the particular load should not become too high to avoid negative effects on the power grid.